In U.S. Pat. No. 3,510,393 assigned to the Assignee of the instant application, a hollow glass fiber strand article is described. Hollow glass fibers of the claimed strands typified in this patent are fibers having outside diameters of 0.0003 to 0.003 inches and having 10 to 65 percent of their volume hollow. The hollow glass fibers described in the aforementioned patent found use in filament wound application, such as a resin reinforcement for radar domes, and may also be used as a resin reinforcement for motor cases, storage tanks and the like. The advantage of hollow glass fibers was to provide a significant improvement in the strength to weight ratio of reinforcing fibers utilized to reinforce composites. Also provided, as stated in the aforementioned patent, was a significant reduction in the dielectric constant of materials reinforced with the hollow glass fibers of the patent over those reinforced with solid fibers.
In U.S. Pat. No. 3,268,313 apparatus suitable for use in manufacturing the hollow glass fibers in the aforementioned patent is shown. Similarly, in U.S. Pat. No. 3,421,873 alternative apparatus and methods are described where fibers can be produced which have intermittent hollowness along their length.
While the aforementioned apparatus, methods and fibers have found some utility in the market place, their use has been limited due to several factors. One factor was the high cost of preparing the hollow glass fibers. This cost was high due to the fact that the apparatus was difficult to operate on a continuous basis without many process interruptions occurring during the formation of the fibers. The manufacturing costs consequently required high selling prices which was a deterrent in the marketplace. Further, the fibers produced by the processes and apparatus described in these patents, while hollow when initially produced from commercial size bushings, after a period of time produced were found to contain a significant large number of solid fibers as well as hollow glass fibers. Still further, the hollow glass fibers produced in a multiple fiber strand had little or no uniformity with respect to the concentricity of the central lumen of the fibers. Stated another way, the K value of the fibers, that is the ratio of the inside diameter of the central lumen of the fiber to the outside diameter of the hollow fiber was found to be very erratic and subject to wide variations in any given strand of fibers produced.
Thus, despite the advent of the processes in the prior art to manufacture hollow glass fibers, the need still exists for processes and apparatus for producing hollow glass fibers of more uniform configuration and quality. Hollow glass fiber strands containing hollow filaments or fibers therein which possess more uniform ratios of inside to outside diameters and better concentricity of the central lumen are still a desired goal.
In accordance with this invention a process for producing improved hollow glass fibers is provided. The fibers made by the process are characterized in their new form by having a better concentricity in the central lumen than heretofore possible and more uniform K values than fibers heretofore produced. This results in a more uniform strength to weight ratio when these fibers are utilized in composites to provide strength to such materials as thermoset and thermoplastic resins. The novel apparatus described to produce the improved hollow fibers is durable, simple in construction and provides consistent fiber quality during the bushing life.